The flea beetle spray decision: 8 steps

Flea beetles

Seed treatment usually provides effective protection through the risk period for flea beetles, but slow-growing crops under intense flea beetle pressure may require foliar insecticide. Here are 8 steps to make the spray decision.

These seedlings have a lot of striped flea beetles, but what about the whole field? Photo credit: Justine Cornelsen
flea beetle defoliation damage on canola plants
CCC flea beetle defoliation graphic shows various levels of defoliation

8 steps to make the right spray decision

Step 1. Know the spray threshold

The action threshold for flea beetles in canola crops is average leaf area loss of 25 per cent or more. Research found that canola with up to 25 per cent leaf area loss will yield the same as canola with no leaf area loss. Yield loss starts after 25 per cent leaf area loss, and the economic threshold is 50 per cent. Researchers set the 25-per-cent “action threshold” because leaf area loss, in a heavy infestation, can escalate quickly from 25 to 50 per cent and beyond. Read the research behind this recommendation.

Exception: Thresholds are guidelines to help farmers make input decisions that provide a return on investment. Canola plants growing slowly and with low vigour could surpass the action threshold very quickly when flea beetles are numerous and aggressive. In hot, dry weather for example. In that situation, growers may need to spray before 25 per cent. Continue through the following checklist to be sure. Canola Digest article with more on this.

Step 2. Assess leaf area loss

With thresholds in mind, check plants in a number of areas of the field to get an average damage level. Scout the newest leaves. Flea beetles tend to prefer fresh new leaves.

If cotyledons are chewed up but newest leaves show very little feeding, plants may be outgrowing the threat or seed treatments may be having an effect (or both).

On the other hand, if first true leaves are not present, flea beetles may be nipping off fresh apical meristem. If this growing point is lost, these leaves could be considered 100 per cent lost lost. Mark those plants (could use the ‘washer’ test) and return in a day or two to check for growth.(More assessment details.)

Step 3. Assess stem damage

Foul weather (wind, cool) drives flea beetles down to leaf undersides and stems. One shallow feeding mark on a stem might not cause lasting damage, but feeding that eats through the stem could be fatal. Plants that die due to stem feeding would have 100 per cent leaf area loss. seedlings that die in this fashion are hard to find and could also be lost for various other reasons, including cutworms, frost and seedling diseases.

If leaf area loss is below thresholds, but flea beetles are present in high numbers and the crop is not advancing, stem feeding may be the reason. In this situation, be more aggressive with the spray decision, especially with plant counts below five per square foot.

Scouting note: Daily scouting needs to happen in areas approaching action threshold. Within two or three days, flea beetle feeding can escalate way beyond threshold, and actually wipe out whole areas of a field, especially in hot, dry and windy conditions.

This graphic represents what 25% and 50% leaf area loss looks like. If a stem is clipped, loss is 100%.

Step 4. Assess flea beetle feeding activity

Are the insects still present in the field and continuing to feed? If it looks like populations are dwindling, a spray may not be necessary. If flea beetles are slow and dopey, it may mean they have ingested seed treatment insecticide and are no longer feeding. Check again to make sure.

Time of day and weather can influence flea beetle activity. Rain will slow or even stop feeding for the time being. Rain can also help the crop more quickly recover.

Step 5. Consider the plant stand

This should influence your approach to the thresholds. With fewer than five plants per square foot, growers can’t afford to lose any plants. With counts of seven or eight plants per square foot), canola fields can afford to lose a couple of plants per square foot without sacrificing harvest yield. (Flea beetle management tips include seed-time decisions to improve the stand.)

Step 6. Check the crop stage

After the four-leaf stage , the threat is likely over because the crop usually has enough plant material to feed flea beetles without compromising growth, and the plants can compensate for feeding better by this stage. If the crop is uneven (some plants are at the four-leaf stage and some are earlier), keep scouting until most plants have at least three or four true leaves.

Exception: If growing conditions (dry, for example) cause slow crop growth at the four-leaf stage, continue to monitor these crops. In 2021, some canola crops at the four-leaf stage had to be sprayed.

Canola Seed Treatments – A stylized diagram
A comprehensive stylized diagram of canola seed treatments.

Step 7. Check canola fields frequently

Seed treatment insecticide starts working when the seed imbibes water and it can remain active for about three to four weeks after that point. If you have enough moisture for seed to imbibe and germinate, you have enough to solubilize the active ingredient and allow it to move into the plant. Flea beetles need to consume some plant tissue to get a dose of the protectant. High levels of flea beetles can overwhelm seed treatments. And in a slow developing crop, seed treatments can lose their protective capacities before the crop reaches the four-leaf stage. Scout often during these first three weeks, and if flea beetle levels are building and are close to thresholds, daily checking for a few days may be required.

Step 8. If spraying, make if effective

Refer to product labels for proper use instructions. No pre-harvest interval concerns exist for products applied at this growth stage, but this could be a deciding factor on which product to use later in the season: specific products are limited to one application per season and a grower may want to keep these in reserve. Read “Improve results from foliar spray” here.

Consider spray-time temperature to improve results. Pyrethroids, the common insecticide group, have restrictions for application in higher temperatures. The label for pyrethroid Decis (active ingredient deltamethrin), for example, says: “DO NOT spray under a strong temperature inversion, or when temperature exceeds 25°C as this will result in a reduction in control.”

Consider spray nozzles to improve results. Tank mixing with herbicide can reduce efficacy of an insecticide because low-drift herbicide nozzles produce a coarse spray droplet that may not provide effective contact and coverage for flea beetle management.

Cotyledons may show significant damage, but if the first leaves are in good shape, the flea beetle threat may have subsided.
Flea beetles have attacked the underside of cotyledons and stem. Credit: Keith Gabert

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